The membranes of PEM fuel cells must be kept in humid conditions in order to achieve high performance and durability. Therefore, if operated at elevated temperatures, fuel cell systems usually require a humidification device for the feed gases air and/or hydrogen. It has been shown that the fuel gas, which is fed to the anode of the fuel cell stack, requires humidification in order to prevent the fuel cell stack from drying at the fuel inlet. Along the internal channels of the fuel cell stack, there is an increase in water content that causes a humidity gradient in the electrolyte membrane, and inhomogeneous power distribution. The inhomogeneous power distribution might lead to hot spots in some areas, and to excessive water accumulation in other areas, which again has a negative affect on performance and durability. Furthermore, humidification devices have several disadvantages, especially for automotive applications of the fuel cell stack, as they are heavy, expensive, and sometimes, due to the water they contain, subject to freezing at low ambient temperatures.
Previous solutions of the humidification problem involved membrane humidifiers and water injection methods, as well as humid gas recirculation. Recirculation methods take advantage of the fact that gases at the fuel cell outlets are humidified with the water produced in the fuel cell, and can be fed back at the fuel cell inlet in order to bring the humidity there without having liquid water involved. A disadvantage is the need for a recirculation pump, the power consumption of the pump and the humidity gradient in a stack along the channel. Also, the switching of oxidizing feed gas between cathode gas inlets and outlets of the fuel cell was proposed in WO/9928985A1. The advantage provided by that system is the better homogeneity of humidity in the fuel cell as the dry feed gas is alternating in one and the other direction in the channel. The feed gas, however, is suggested to be the oxidant, and is dry, which might lead to performance degradation at both gas inlets. Accordingly, it is desirable in the art to provide a method of providing homogeneous membrane humidification without having liquid water involved in the process and without requiring additional pumps or the use of additional power.